Helper functions to create and warm-up the snapshot blob were
duplicated in various spots (mksnapshot, inspector tests, serializer
cctests). This merges all of these into a single helper function
family declared in snapshot.h.

Bug: v8:9189, chromium:957029
Change-Id: I2d8d6fd8e955ffffd7d805c199d4a858500af588
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1598695
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61576}